1. Field of the Invention
The present invention relates to an objective lens element for use in an optical pickup device that performs at least one of recording, reproducing, and erasing of information on an optical information storage medium.
2. Description of the Background Art
In recent years, an optical pickup device for high-recording-density optical disc (e.g., Blu-ray Disc (registered trademark)) is desired to be reduced in thickness, similarly to conventional ones used for CD and DVD. Thickness reduction of the optical pickup device results in size reduction and thickness reduction of an apparatus equipped with the optical pickup device, leading to expansion of the range of apparatuses that can be equipped with the optical pickup device.
One of optical components that greatly contribute to the thickness of the optical pickup device is an objective lens. Due to limitations of optical arrangement, the thickness of the objective lens directly influences the thickness of an optical pickup. Thus, in order to achieve thickness reduction of the optical pickup device, it is necessary to reduce the thickness of the objective lens.
In the case of an objective lens for BD, in order to compensate an off-axis aberration and the like in the range of high NA, the lens thickness tends to be large. The expression “the lens thickness is large” means that the on-axis lens thickness is large as compared to that of objective lenses for CD and DVD having relatively low NAs. Hereinafter, a biconvex single lens element which is advantageous in terms of optical performance and cost will be described as an example.
An objective lens optical surface on a light source side has to greatly bend an incident light beam, and thus in general has great power and tends to have a great amount of sag. In addition, an objective lens optical surface on a disc side also has convex power, and thus the lens thickness decreases toward the outer periphery. Here, an optical surface includes a refractive surface formed by an aspheric surface, a diffractive surface, and a phase step surface.
When the lens thickness is reduced as much as possible, a sufficient edge thickness cannot be ensured near the lens effective diameter and at the outer periphery. In addition, unless a certain amount of edge thickness is ensured, a crack occurs during a lens molding process, and stable lens molding is difficult.
For example, Japanese Laid-Open Patent Publication Nos. 2007-334928, 2007-334929, and 2007-334930 disclose technology in which, in a high-NA objective lens, a region thicker than the thickness in the optical axis direction within the effective diameter is provided in an outer peripheral portion outside the effective diameter.
However, the above prior art documents merely disclose objective lenses made of resin, and the disclosed technology cannot be applied to an objective lens made of glass. The reason is that due to the difference in producing method between resin lens and glass lens, methods of machining molds therefor are different from each other. Therefore, a discontinuous surface shape as described in the above prior art documents cannot be applied to a glass lens. In addition, in the above prior art documents, the thickness of the outer peripheral portion in the optical axis direction is increased in order to improve transfer of the mold shape to enhance the lens aberration performance and in order to cause the outer peripheral portion to serve as a member which prevents collision with an optical disc.